Titanium impregnated silica-chromium catalysts

ABSTRACT

In accordance with one embodiment of this invention, a silica hydrogel is prepared and dried in the presence of a pore preserving agent after which a titanium compound is anhydrously incorporated therewith. In accordance with the second embodiment of this invention, a silica hydrogel or a silica xerogel is impregnated with an aqueous organic acid solution of a substituted or unsubstituted titanium acetylacetonate. The invention allows the use of conventional drying to produce a silica exhibiting characteristics associated with azeotrope dried titanium-silica for use as a support for chromium catalysts. The resulting catalysts are broadly applicable in the polymerization of olefins and are of particular utility in the slurry polymerization of olefins, particularly predominantly ethylene monomer.

BACKGROUND OF THE INVENTION

This invention relates to silica supported chromium catalysts containingtitanium.

Supported chromium oxide catalysts have been used for many years in thepolymerization of olefins to solid polymers. One widely used method ofproducing such catalysts involves precipitating a silica hydrogel,impregnating the hydrogel with an aqueous solution of a chromiumcompound and drying. Such procedure gives a silica which inherently hassufficient strength to resist severe shrinkage of the pores duringsimple drying techniques such as oven drying, tray drying, spray dryingor drying under a heat lamp. Such catalyst is simple and inexpensive toproduce and gives outstanding results in solution polymerization ofolefins to give normally solid polymer.

There is an economic advantage in some instances to producing olefinpolymers in a slurry as opposed to a solution system. However, certaincontrol operations which are easily carried out in the solution processare considerably more difficult in the particle-form or slurry process.For instance in the solution process, control of the molecular weightcan be effected by changing the temperature, with lower molecular weight(higher melt flow) being obtained at the higher temperature. However, inthe slurry process, this technique is inherently limited since anyeffort to increase the melt flow to any appreciable extent by increasingthe temperature causes the polymer to go into solution and thus destroysthe slurry process.

It is known that titanium affects the polymerization activity of silicasupported chromium catalysts in a way that is of special importance inslurry polymerizations. However, when titanium is coprecipitated withthe silica, it produces a hydrogel which does not have sufficientstrength to resist serious collapse of the pores during simple dryingsuch as spray drying. Similarly, in accordance with the prior art, if anaqueous solution of a titanium compound is added to the dried silica,the pores are damaged. Accordingly, in order to take full advantage ofthe improvement which can be imparted to the melt index capabilitythrough the use of titanium in accordance with the prior art, thetitanium had to be coprecipitated with the silica and the resultinghydrogel (cogel) dried by a more expensive azeotrope distillation orwashing with a liquid oxygen-containing water soluble organic compound.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a titanium-containingsilica supported chromium catalyst which can be dried in a conventionalmanner and yet which exhibits the characteristics associated withazeotrope dried titanium-silica coprecipitated catalysts.

It is a further object of this invention to provide a catalyst suitablefor use in slurry polymerization systems; and

It is yet a further object of this invention to provide an improvedolefin polymerization process.

There are two embodiments to this invention. In accordance with thefirst embodiment, silica hydrogel is prepared and dried in the presenceof a pore preserving agent after which a titanium compound isanhydrously incorporated therewith.

In accordance with the second embodiment of this invention, a silicahydrogel or a silica xerogel is impregnated with an aqueous organic acidsolution of a substituted or unsubstituted titanium acetylacetonate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

In accordance with embodiment 1, a silica hydrogel containing a porepreserving agent is dried and thereafter the titanium compound isintroduced in an anhydrous manner. The silica hydrogel can be preparedin a conventional manner, for instance, by contacting an aqueous acidsolution with an aqueous solution of an alkali metal silicate asdisclosed in Witt, U.S. Pat. No. 3,900,457, issued Aug. 19, 1975, thedisclosure of which is hereby incorporated by reference. Preferably, thealkali metal silicate is added to the acid.

Prior to drying, the hydrogel must contain a pore preserving agent. Thesilica pore structure preserving agents can be selected from amongorganic silicon compounds, e.g., triarylsilanols, described inapplication Ser. No. 914,258, filed June 9, 1978 now U.S. Pat. No.4,190,457, the disclosure of which is hereby incorporated by reference;oxygen-containing organic compounds selected from among polyhydricalcohols, mono- and dialkyl ethers of ethylene glycol andpoly(alkylene)glycol as disclosed in McDaniel, U.S. Pat. No. 4,169,926,issued Oct. 2, 1979, the disclosure of which is hereby incorporated byreference and surfactants. U.S. Pat. No. 4,169,926 also disclosessuitable anionic, cationic and nonionic surfactants. This patent alsodiscloses combinations of the oxygen-containing organic compounds with anormally liquid hydrocarbon, e.g., n-heptane, kerosene, and, optionally,a surfactant, which are also suitable pore preserving agents. Thenonionic surfactants are preferred.

Said organic silicon compounds have the structure

    R.sub.n SiA.sub.4-n

wherein n is an integer of 2 or 3, and wherein each R is a saturated orunsaturated hydrocarbon group wherein each R can be the same ordifferent and wherein A is selected from the group consisting of hydroxyradicals, halides and alkoxy radicals in which the alkyl group thereincontains from 1 to about 10 carbon atoms.

Preferably, R is selected from the group consisting of alkyl radicals offrom 4 to about 12 carbon atoms, alicyclic radicals of from 4 to about12 carbon atoms, aryl radicals of from 6 to about 24 carbon atoms, andhydrocarbyl-substituted aryl radicals such as alkylaryl andcycloalkylaryl of from 6 to about 24 carbon atoms.

The pore preserving agents can also include certain inorganic andorganic acids used at a specific level of pH. Specifically the hydrogelis contacted with an inorganic or organic acid in an amount sufficientto impart to the mixture a pH ranging generally from about 0 to about3.5, more specifically from about 0 to 3. About 2.2 or below ispresently believed to be preferred.

Inorganic acids employable are those which are water soluble,sufficiently ionized to produce the pH level required in the hydrogels,and do not have a deleterious effect on the silica or in the end useapplication. Specific but nonlimiting acids can be selected from amonghydrochloric acid, hydrobromic acid, hydriodic acid, nitric acid,sulfamic acid, sulfuric acid, orthophosphoric acid and iodic acid.

Organic acids generally employable in this invention are those whichexhibit the same requirements as the inorganic acids. Specific butnonlimiting examples include acetic acid, formic acid, tartaric acid,citric acid, maleic acid, malic acid, malonic acid, succinic acid,gluconic acid, diglycolic acid, ascorbic acid, cyclopentanetetracarboxylic acid, and benzenesulfonic acid.

In general, those organic acids meeting the requirements of watersolubility, stability, acid strength, nondeleterious action as describedbefore also have pK values of about 4.76 or less as disclosed in Lange'sHandbook of Chemistry, 11th Edition (1973), Tables 5-7, 5-8. In otherwords their acid strength is equal to or greater than that of aceticacid.

Acids such as sulfuric acid and hydrochloric acid are generallypreferred, however, because of their ready availability, relatively lowcost, great acid strength, and efficacy in the process.

Specific examples of preferred pore preserving agents are apolysiloxane-polyoxyalkylene copolymer, a polyethoxylated sorbitolmonolaurate, and a polyethoxylated t-octyl phenol.

The pore preserving agent can be incorporated in one of the ingredientsused to make the silica hydrogel, however, it is preferably incorporatedinto the hydrogel after the washing step since this avoids loss of theagent during the washing step.

The hydrogel containing the pore preserving agent is then conventionallydried using an air oven, spray drying, tray drying, vacuum oven dryingor drying under a heat lamp, for instance. Conventional dryingtemperatures of room temperature to 425° C. or higher can be used tothus remove free water and produce a xerogel. With spray drying theincoming air can be up to 425° C. although the catalyst does not getthat hot.

The titanium compound is then incorporated under anhydrous conditionsinto the xerogel. This can be done by utilizing a nonaqueous solution ofa titanium compound such as a titanium alkoxide, generally at anelevated temperature, to deposit the titanium compound. Alternatively,any desired titanium compound can simply be added to the xerogel duringthe first part of the activation (calcining) step in a fluidized bed.The titanation can also be separately effected at about 100° to about200° C. in a fluidized bed, if desired.

Further with respect to the pore preserving agent, when the porepreserving agent is an oxygen-containing organic compound, the weightratio of oxygen-containing organic compound to hydrogel employed inmaking the catalyst can range from about 5:1 to about 0.5:1.

When both an oxygen-containing organic compound and a normally liquidhydrocarbon are employed with the hydrogel, the weight ratio ofhydrocarbon to organic compound can vary from about 20:1 to about 0.5:1.

When employing one or more surfactants with thehydrocarbon/oxygen-containing organic compound, generally about 0.1 toabout 5 weight percent surfactant is used based on the weight ofhydrocarbon/oxygen-containing organic compound.

When a surfactant or an organic silicon compound of said Ser. No.914,258 is employed as the silica pore structure preserving agent, theweight ratio of hydrogel to surfactant or organic silicon compound canrange from about 20:1 to about 500:1, preferably from about 40:1 toabout 100:1.

Sufficient treating time is allotted to obtain contact of the variousadded components with the hydrogel or dried gel. Generally, timesranging from about 30 seconds to about 10 hours, preferably 15 minutesto 120 minutes are adequate. The treated hydrogel is then dried asdescribed above to remove the liquids and the composite is thenactivated at an elevated temperature.

The activation can be carried out in a conventional manner by calciningat an elevated temperature, generally from about 400° C. to 1100° C. ina dry atmosphere containing oxygen, generally in air. The resultingcatalyst contains at least a portion of the chromium in the hexavalentstate. Alternatively, the catalyst can be activated in accordance withthe reduction/reoxidation procedures disclosed in McDaniel and Welch,U.S. Pat. No. 4,151,122, which issued Apr. 24, 1979, the disclosure ofwhich is hereby incorporated by reference. Use of this activationtechnique further enhances the capability to produce high melt indexpolymers exhibiting superior stress crack resistance.

Embodiment 2

In this embodiment, a silica is impregnated with an aqueoustitanium-containing composition prepared as described hereinbelow from atitanium compound of the following structural formula: ##STR1## whereinthe R and R' groups are the same or different and are selected from 1-7carbon atom alkyl radicals. When all of the R groups are methyl and theR' is isopropyl the compound is diisopropoxy titanium acetylacetonate.The preferred titanium components are the acetylacetonates, i.e., whereR is methyl most preferably those where R' is isopropyl or butyl.

If desired, a pore preserving agent as described in embodiment 1 can beused in the hydrogel to further enhance the beneficial qualities of thecatalyst and such is preferred. That is, for instance, an alkali metalsilicate can be added to an acid to precipitate a silica hydrogel whichis then washed and impregnated with a water soluble chromium compoundand thereafter dried in a conventional manner such as spray drying, traydrying, oven drying or drying with a heat lamp as described inembodiment 1. Thereafter, the thus formed xerogel is impregnated withthe aqueous titanium composition. Alternatively the aqueous titaniumcomposition can be combined with the hydrogel before drying, againeither with or without a pore preserving agent in the hydrogel, afterwhich the hydrogel is dried and activated as described in embodiment 1.

It is essential in accordance with Embodiment 2 that the titaniumcompound first be mixed with an anhydrous or at least essentially waterfree organic acid and thereafter diluted with water. The exact role ofthe organic acid is not known with certainty. It is not simply a matterof solubilizing the titanium compound (which is insoluble in water)since it does not work to dissolve the titanium component in othersolvents such as acetone since a precipitate forms when such a mixtureis diluted with water. By essentially water free organic acid is meantone having less than 5 percent water, preferably less than 1 percent.The volume ratio of essentially water free organic acid to titaniumacetylacetonate must be greater than 0.5:1. Preferably it is from 1:1 to10:1 more preferably about 2:1. The amount of acid can be greater than10:1 if desired so far as operability is concerned, but generally asmaller amount is desired for practical reasons. The amount must begreater than 0.5:1, however, since the invention does not operatesatisfactorily with only one part acid for two parts of the titaniumcompound. The acid can be added to the titanium component or vice versa.

After the organic acid and titanium compound have been mixed, theresulting titanium-containing composition is diluted with water to givethe aqueous titanium-containing composition. Preferably, a ratio ofwater:titanium-containing composition of about 7:3 is used although theratio can be from about 1:1 to as much as 10:1 or greater. The water canbe added to the composition or vice versa. When the aqueoustitanium-containing composition is used to impregnate a xerogel, thethus-impregnated xerogel can then be directly activated as in embodiment1 or the water removed by simple evaporation and the dry compositeactivated.

In all embodiments, the chromium component of the catalyst can becoprecipitated with the silica or added by means of an anhydroussolution of a chromium compound soluble in nonaqueous solvents such ashydrocarbons to the xerogel but preferably the chromium is introduced bymeans of incorporating an aqueous solution of a water soluble chromiumcompound with the hydrogel. This is preferably done after the hydrogelis washed with water to remove alkali metal ions. The water solublechromium compound can be selected from among chromium acetate, chromicnitrate, chromic sulfate, chromous sulfate, chromium trioxide, ammoniumchromate, and other soluble chromium compounds. Chromium acetate andCrO₃ are preferred.

The amount of chromium compound employed in making the catalyst issufficient to provide from about 0.001 to about 10 weight percent,preferably 0.1 to 5 weight percent chromium based on the weight of theactivated catalyst.

The amount of titanium compound employed in making the catalyst issufficient to provide from about 0.1 to 10, preferably 1.5 to 5.5 weightpercent titanium based on the weight of the activated catalyst.

The catalyst of this invention resembles coprecipitated silica-titaniumcatalyst in pore volume, in chromium content, and in the titaniumcontent although the process for making the catalyst is substantiallysimplified and substantially less expensive.

The catalyst of this invention can be used to polymerize at least onemono-1-olefin containing 2 to 8 carbon atoms per molecule. The inventionis of particular applicability in producing ethylene homopolymers andcopolymers from mixtures of ethylene and one or more comonomer selectedfrom 1-olefins containing 3 to 8 carbon atoms per molecule. Exemplarycomonomers include aliphatic 1-olefins such as propylene, 1-butene,1-hexene, and higher 1-olefins and conjugated or nonconjugateddiolefins, such as 1,3-butadiene, isoprene, piperylene,2,3-dimethyl-1,3-butadiene, 1,4-pentadiene, 1,7-hexadiene, and mixturesthereof. Ethylene copolymers preferably constitute at least about 90,more preferably 95 to 99 mole percent polymerized ethylene units.Ethylene, propylene, 1-butene, and 1-hexene are especially preferred.

The polymers can be prepared from the catalyst of this invention bysolution polymerization, slurry polymerization, and gas phasepolymerization technique using conventional equipment and contactingprocesses. However, the catalyst of this invention is particularlysuitable in slurry polymerizations for the production of high melt indexpolymers in the absence of molecular weight modifiers, such as hydrogen.The slurry process is generally carried out in an inert diluent such asa paraffin, aromatic or cycloparaffin hydrocarbon. For predominantlyethylene polymers, a temperature of about 66° to 110° C. is employed.Hydrogen or other molecular weight modifiers can be used, of course, ifdesired. Also conventional cocatalysts and adjuvants and activators canbe utilized, if desired.

EXAMPLES Example I

This exemplifies embodiment 1 of the invention.

Hydrogel containing about 20 weight percent solids was prepared byintroducing an alkali metal silicate into an acid. The hydrogel wasadmixed with sufficient aqueous chromium trioxide solution to provide 1weight percent chromium based on the dry composite.

A series of catalysts was prepared from a conventionally spray driedcomposite formed by admixing a portion of a hydrogel with sufficientaqueous chromium (III) acetate solution to provide 1 weight percentchromium based on the dry composite and 2 volume percent based on thehydrogel of a liquid nonionic surfactant described as a polyethoxylatedt-octylphenol, commercially available from Alcolac, Inc., Baltimore, Md.under the trademark Siponic F-300.

Individual portions of the spray dried composite were slurried in about50 ml of dry n-hexane and a specified amount of n-hexane solution oftitanium tetraisopropoxide containing 16 g titanium/100 ml solution wasadded and mixed with the slurry. The solvent was evaporated off byutilizing a hot plate and the product was converted into an activecatalyst by heating it for 5 hours at 649° C. with dry air in afluidized bed (Runs 1-6).

Individual portions of the spray dried composite containing a specifiedamount of titanium tetraisopropoxide added to a hexane slurry of thecomposite and dried as described earlier in this example were activatedsequentially in a carbon monoxide atmosphere and an air atmosphere underfluid bed conditions. Each sample was fluidized in dry CO at 815° C. for3 hours, the activator was flushed with nitrogen to remove the CO as thetemperature was lowered to 705° C., and then fluidization with dry airat 705° C. was continued for 2 hours. The activated catalyst wasrecovered and stored as before pending the polymerization testing (Runs7-12).

Other individual portions of the spray dried composite were fluidized inthe activator with dry nitrogen at about 200° to 300° C. for 1 hour as aspecified amount of titanium tetraisopropoxide was dripped into thefluidized bed. The titanium addition required about 10 minutes. Thetemperature was increased to about 550° C. in the nitrogen stream, thenitrogen was cut off and dry air substituted and the temperature raisedto 650° C. over a 30 minute period and held at 650° C. for 5 hours. Theactivated catalysts were recovered and stored in a dry atmosphere untilneeded for polymerization testing (Runs 13-16).

A portion of each activated catalyst was used in ethylene particle formpolymer by conducting the polymerization at 107° C. in the presence of567 g of isobutane as diluent and ethylene at a nominal reactor pressureof 3.9 MPa for a time sufficient to produce about 3000 g polymer per gcatalyst. Polymer was recovered and melt index was determined asdescribed before. Each MI value was corrected to a common productivitylevel of 3000 g polymer per g catalyst so that a valid comparison of thevalues could be made. The catalyst weight and titanium level of eachcatalyst employed and the results obtained are given in Table IA, IB andIC. The calculated percent melt index improvement values are based onthe melt index of the respective control run values.

                                      TABLE IA                                    __________________________________________________________________________    Ethylene Polymerization, 107° C., 3.9 MPa                              Titanium Compound Added By Nonaqueous Impregnation                                                          Percent                                         Catalyst    Polymer           Calculated                                      Run                                                                              Wt. %                                                                             Cat. Wt.                                                                           Yield                                                                              Productivity                                                                           Melt                                                                              Melt Index                                                                           HLMI                                     No.                                                                              Ti  g    g    g polymer/g cat.                                                                       Index                                                                             Improvement                                                                          MI  Remarks                              __________________________________________________________________________    1  0    0.0590                                                                            179  3030      0.68                                                                             --.sup.(a)                                                                           75  control                              2  1   .0545                                                                              173  3170     1.9 180    55  invention                            3  2   .0525                                                                              165  3140     1.6 140    69  invention                            4  3   .0580                                                                              164  2830     1.2  76    69  invention                            5  4   .0630                                                                              210  3330     1.4 110    77  invention                            6  5   .0750                                                                              202  2690     1.9 180    79  invention                            __________________________________________________________________________     .sup.(a) A dash signifies no entry is needed.                            

                                      TABLE IB                                    __________________________________________________________________________    Ethylene Polymerization, 107° C., 3.9 MPa                              Titanium Compound Added By Nonaqueous Impregnation                            Catalyst Activated Sequentially in CO and Air                                                               Percent                                         Catalyst    Polymer           Calculated                                      Run                                                                              Wt. %                                                                             Cat. Wt.                                                                           Yield                                                                              Productivity                                                                           Melt                                                                              Melt Index                                                                           HLMI                                     No.                                                                              Ti  g    g    g polymer/g cat.                                                                       Index                                                                             Improvement                                                                          MI  Remarks                              __________________________________________________________________________     7 0    0.0595                                                                            187  3140      0.81                                                                             --.sup.(a)                                                                           81  control                               8 1   .0690                                                                              212  3070     1.8 120    71  invention                             9 2   .0575                                                                              197  3430     5.7 600    52  invention                            10 3   .0500                                                                              176  3520     5.1 530    46  invention                            11 4   .0460                                                                              157  3410     5.5 580    53  invention                            12 5   .0535                                                                              142  2650     11  1260   45  invention                            __________________________________________________________________________     .sup.(a) A dash signifies no entry is needed.                            

                                      TABLE IC                                    __________________________________________________________________________    Ethylene Polymerization, 107° C., 3.9 MPa                              Titanium Compound Added During Catalyst Activation                                                          Percent                                         Catalyst    Polymer           Calculated                                      Run                                                                              Wt. %                                                                             Cat. Wt.                                                                           Yield                                                                              Productivity                                                                           Melt                                                                              Melt Index                                                                           HLMI                                     No.                                                                              Ti  g    g    g polymer/g cat.                                                                       Index                                                                             Improvement                                                                          MI  Remarks                              __________________________________________________________________________    13 0   0.0815                                                                             230  2820     0.25                                                                              --.sup.(a)                                                                           88  control                              14 1   .0490                                                                              148  3020     .27  8     74  invention                            15 3   .0445                                                                              148  3330     .71 184    73  invention                            16 5   .0460                                                                              105  2280     .86 244    63  invention                            __________________________________________________________________________     .sup.(a) A dash signifies no entry is needed.                            

Inspection of the results given in Tables IA, IB, IC shows thattitanation of the dry silica-chromium compound composite can beaccomplished either with a hydrocarbon solution of the titanium compoundby impregnation or with neat addition of the titanium compound to thefluidized bed during the preliminary phase of the activation cycle giventhe catalyst. Active catalysts are formed which exhibit modest tomarkedly superior melt index capabilities compared to the controlcatalysts. When catalysts prepared according to the invention aresequentially activated in carbon monoxide and air, then their melt indexcapability is even more substantially improved, particularly when thetitanium content of the catalysts range from about 2 to about 5 weightpercent.

Example II

This exemplifies embodiment 2 of the invention.

A series of catalysts was prepared by admixing 12.7 g portions of a dry,commercially available silica-chromium oxide polymerization catalystcontaining 1 weight percent chromium as the oxide with the components,when employed, described below. Each sample was dried overnight in avacuum oven at 100° C., then activated in a fluidized bed with dry airat 760° C. for 5 hours. No pore preserving agents were used.

Catalyst 1, control; no titanium compound added.

Catalyst 2, invention; dry control catalyst was admixed with 38.1 ml ofa solution formed by dissolving 10 ml (9.9 g) diisopropoxy titaniumacetylacetonate [Ti(ACAc)₂ ] in 20 ml (21 g) of glacial acetic acid(HAc) and then diluting with 70 ml of water. The water to HAc volumeratio was then about 3.5 to 1. The amount of titanium compound added wassufficient to provide 3 weight percent titanium based on the drycomposite.

Catalyst 3, invention; dry control catalyst was admixed with 19 ml of asolution formed by dissolving 10 ml of Ti(AcAc)₂ in 20 ml of HAc andthen diluting with 20 ml of water. The water to HAc volume ratio wasthen 1 to 1. The amount of titanium compound was sufficient to provide 3weight percent titanium based on the dry composite.

Catalyst 4, invention; duplicate of catalyst 3 to indicatereproducibility of the catalyst forming method.

Catalyst 5, control; dry catalyst was admixed with 38.1 ml of a solutionformed by dissolving 10 ml of Ti(ACAc)₂ in 20 ml (16 g) of methyl ethylketone (MEK) and then diluting with 70 ml of water. The water to MEKvolume ratio was then about 3.5 to 1. The amount of titanium compoundadded was sufficient to provide 3 weight percent titanium based on thedry composite.

Samples of the activated catalysts were employed in ethylenepolymerization exactly as described in the first example. The catalystweights used and the results obtained are given in Table II. Calculatedpercent melt index improvement is based on the melt index obtained incontrol run 1.

                                      TABLE II                                    __________________________________________________________________________    Ethylene Polymerization, 107° C., 3.9 MPa                              Aqueous Titanation of Dry Catalyst                                                                          Percent                                         Catalyst    Polymer           Calculated                                      Run                                                                              Wt. Wt. %                                                                              Yield                                                                              Productivity                                                                           Melt                                                                              Melt Index                                                                           HLMI                                     No.                                                                              g   Ti   g    g polymer/g cat.                                                                       Index                                                                             Improvement                                                                          MI  Remarks                              __________________________________________________________________________    1  0.0450                                                                            0    216  4800     0.35                                                                              --.sup.(a)                                                                           84  control                              2  .0456                                                                             3    229  5020     0.40                                                                              14     81  invention.sup.(b)                    3  .0408                                                                             3    203  4980     0.70                                                                              100    68  invention.sup.(b)                    4  .0477                                                                             3    260  5450     0.63                                                                              80     86  invention.sup.(b)                    5  .0477                                                                             3    231  4860     0.34                                                                              none   95  control.sup.(c)                      __________________________________________________________________________     .sup.(a) A dash signifies no entry is nedded.                                 .sup.(b) Glacial acetic acid and water solution used with titanium            compound.                                                                     .sup.(c) Methyl ethyl ketone and water solution used with titanium            compound.                                                                

The results in Table II show that a dry silica-chromium oxide catalystcan be successfully aqueously titanated with a solution consisting ofdiisopropoxy titanium acetylacetonate dissolved in a glacial acetic acidwater mixture. Catalysts recovered and activated produce ethylenepolymers having melt index values ranging from 14 to 100 percent better(runs 2-4) than the untitanated control catalyst of run 1. When methylethyl ketone is substituted for acetic acid in preparing the catalystthe melt index results in control run 5 demonstrate no improvement inmelt index capability over the control catalyst of run 1.

It is believed similar good results would be obtained by incorporatingthe aqueous titanium composition in the hydrogel and such would bepreferred for economic reasons.

While this invention has been described in detail for the purpose ofillustration, it is not to be construed as limited thereby but isintended to cover all changes and modifications within the spirit andscope thereof.

I claim:
 1. A process for producing a chromium-containing catalyst comprising forming a silica hydrogel containing a pore preserving agent selected from (1) organic silicon compounds having the structure R_(n) SiA_(4-n) wherein n is an integer of 2 or 3, and wherein each R is a saturated or unsaturated hydrocarbon group wherein each R can be the same or different and wherein A is selected from the group consisting of hydroxy radicals, halides and alkoxy radicals in which the alkyl group therein contains from 1 to about 10 carbon atoms, (2) water soluble inorganic acids which are sufficiently ionized to produce a pH of 0 to 3.5, (3) organic acids having an acid strength equal to or greater than that of acetic acid, and (4) anionic, cationic or non-ionic surfactants; drying said hydrogel by means of air oven drying, spray drying, tray drying, vacuum oven drying or drying under a heat lamp to form a xerogel; and anhydrously incorporating a titanium compound into said xerogel, and wherein said chromium is introduced by either coprecipitating same with said silica hydrogel, by combining an aqueous solution of a water-soluble chromium compound with said hydrogel, or adding anhydrous solution of a chromium compound soluble in non-aqueous solvents to said xerogel.
 2. A process according to claim 1 wherein said chromium in the form of a compound soluble in nonaqueous solvents is anhydrously impregnated onto said xerogel.
 3. A method according to claim 1 wherein said hydrogel is washed and after said washing said pore preserving agent is admixed therewith.
 4. A method according to claim 3 wherein said chromium is added as an aqueous solution of a water soluble chromium compound to said hydrogel.
 5. A method according to claim 4 wherein said chromium is incorporated in the form of a compound selected from CrO₃ and chromium acetate.
 6. A method according to claim 3 wherein said chromium-containing catalyst contains from about 0.5 to about 4 weight percent chromium and from 1.5 to 5.5 weight percent titanium based on the total weight of the final dried catalyst.
 7. A method according to claim 3 wherein said titanium is incorporated by impregnating said xerogel with a nonaqueous solution of a titanium compound.
 8. A method according to claim 7 wherein said titanium compound is added in a hydrocarbon solution.
 9. A method according to claim 8 wherein said titanium compound is titanium tetraisopropoxide.
 10. A method according to claim 3 wherein said titanium compound is added neat to said xerogel during a subsequent activation process.
 11. A method according to claim 1 wherein said pore preserving agent is selected from a polysiloxane-polyoxyalkylene copolymer, a polyethoxylated sorbitol monolaurate, and a polyethoxylated t-octylphenol.
 12. A method according to claim 3 wherein said hydrogel is spray dried.
 13. A method according to claim 12 wherein said pore preserving agent is selected from a polysiloxane-polyoxyalkylene copolymer, a polyethoxylated sorbitol monolaurate, and a polyethoxylated t-octylphenol, said titanium compound is titanium tetraisopropoxide and wherein said titanium compound is added from a hydrocarbon solution.
 14. A method according to claim 1 wherein said xerogel is activated by reduction and reoxidation.
 15. A catalyst produced by the method of claim
 14. 16. A catalyst produced by the method of claim
 1. 17. A process for producing a chromium-containing catalyst comprising forming a silica hydrogel and thereafter removing water to form a xerogel, said silica in either the hydrogel stage or the xerogel stage being impregnated with an aqueous titanium-containing composition formed by mixing an essentially water-free organic acid and a titanium compound of the formula ##STR2## wherein the R and R' groups are the same or different and are selected from 1-7 carbon atom alkyl radicals to give a titanium-containing composition and thereafter diluting said titanium-containing composition with water to give said aqueous titanium-containing composition, and wherein said chromium is introduced by either coprecipitating same with said silica hydrogel, by combining an aqueous solution of a water-soluble chromium compound with said hydrogel, or adding an anhydrous solution of a chromium compound soluble in a non-aqueous solvent to said xerogel.
 18. A method according to claim 17 wherein said organic acid and said titanium compound are mixed in a ratio within the range of 1:1 to 10:1 of acid:titanium compound to form said titanium-containing composition.
 19. A method according to claim 18 wherein said titanium-containing composition is diluted with said water in a ratio within the range of 1:1 to 10:1 water:titanium-containing composition to give said aqueous titanium-containing composition.
 20. A method according to claim 17 wherein said aqueous titanium containing-containing composition is added to said silica at the hydrogel stage.
 21. A method according to claim 17 wherein said aqueous titanium-containing composition is added to said silica in the xerogel stage and thereafter the thus titanium impregnated xerogel is dried.
 22. A method according to claim 21 wherein said silica is formed from a hydrogel also containing a pore preserving agent selected from organic silicon compounds, oxygen-containing organic compounds, acids and surfactants.
 23. A method according to claim 17 wherein said R groups are methyl.
 24. A method according to claim 17 wherein said silica is made by adding an alkali metal silicate to an acid to form a hydrogel, washing the hydrogel with water, impregnating the thus washed hydrogel with an aqueous solution of a chromium compound selected from CrO₃ and chromium acetate and spray drying, said aqueous titanium-containing composition being added to the washed hydrogel.
 25. A method according to claim 24 wherein said chromium-containing catalyst contains from about 0.5 to about 4 weight percent chromium and 1.5 to 5.5 weight percent titanium based on the total weight of the final dried catalyst and wherein the titanium compound is diisopropoxy titanium acetylacetonate and wherein said essentially water free organic acid is glacial acetic acid.
 26. A method according to claim 25 wherein said thus impregnated silica is activated by reduction and reoxidation.
 27. A catalyst produced by the method of claim
 26. 28. A catalyst produced by the method of claim
 17. 